1. Field of the Invention
The present invention relates to an ink jet recording apparatus and in particular, to an ink jet recording head drive method for recording characters and images by discharging ink droplets from a nozzle and an apparatus thereof.
2. Description of the Related Art
Conventionally, there is known a drop-on-demand type ink jet apparatus in which an electro-mechanical converter such as a piezoelectric actuator is used to generate a pressure wave (accoustic wave), which serves to eject an ink droplet from a nozzle connected to a pressure generation chamber. This type of ink jet recording head drive method is disclosed, for example, in Japanese Patent Publication (examined) 53-12138. This type of ink jet recording head is shown in FIG. 25 as an example.
Referring to FIG. 25, a pressure generation chamber 100 is connected to a nozzle 101 for discharging ink and an ink supply path 103 for introducing ink from an ink tank (not depicted) via a common ink chamber 102. Moreover, at the bottom of the pressure generation chamber 100, a diaphragm 104 is provided. When discharging an ink droplet, this diaphragm 104 is displaced by a piezoelectric actuator 105 (electro-mechanical converter) provided outside the pressure generation chamber 100, so as to generate a volume change of the pressure generation chamber 100, thus generating a pressure wave in the pressure generation chamber 100. This pressure wave ejects a portion of ink from the pressure generation chamber 100 outside via the nozzle 101 and the ink droplet 106 flies to a recording medium such as a recording paper to form a recording dot. The formation of recording dot is repeatedly performed according to an image data, so as to record a character and an image on the recording paper.
In order to obtain a high quality image using this type of ink jet recording head, it is necessary to set the diameter of the ink droplet 106 very small. That is, in order to obtain a smooth image without feeling of the respective droplets, it is necessary to make the recording dot (pixel) as small as possible. For this, the diameter of the ink droplet ejected should be set very small. Normally, when the dot diameter is equal to or smaller than 40 micrometers, the image quality is remarkably improved. The ink droplet diameter and the dot diameter depend on the ink droplet flying speed (droplet speed), ink characteristic (such as viscosity and surface tension), the type of the recording paper. Normally, the dot diameter is twice as much as the ink droplet diameter. Accordingly, in order to obtain a dot diameter of 40 micrometers or less, the ink droplet should have a diameter of 20 micrometers or less. It should be noted that in the explanation below, the droplet diameter represents a total ink amount ejected by one eject operation (including a satellite shown by 106xe2x80x2 in FIG. 25) which is preceded by a corresponding spherical droplet.
In order to reduce the ink droplet diameter, the nozzle 101 should have a reduced diameter. However, considering technical limits and reliability such as a problem of clogging, the nozzle diameter practically has a lower limit of 25 micrometers. It is difficult to obtain an ink droplet of the 20 micrometers level only by reducing the nozzle diameter. To cope with this, an attempt has been made to reduce the ink droplet diameter through the recording head drive method and several effective methods have been suggested.
As a drive method for discharging a very small droplet by the ink jet recording head, for example, Japanese Patent Publication (unexamined) 55-17589 discloses a drive method for temporarily expanding the pressure generation chamber immediately before eject and an ink surface formed by reserved ink in a nozzle opening (hereinafter, referred to as meniscus) is pulled into the pressure generation chamber and then ejected. FIG. 26(a) shows an example of a drive voltage waveform used in this type of drive method. It should be noted that the relationship between the drive voltage and operation of the piezoelectric actuator 105 varies depending on the structure of the actuator 105 and polarization direction. In the explanation given below, it is assumed that increase of the drive voltage decreases the volume of the pressure generation chamber 100 while decrease of the drive voltage increases the volume of the pressure generation chamber 100.
The drive voltage waveform of FIG. 26(a) consists of a first voltage change process 1 for expanding the pressure generation chamber 100 so as to pull the meniscus from the nozzle opening into the pressure generation chamber 100 and a second voltage change process 2 that compresses the pressure generation chamber 100, so as to eject an ink droplet.
FIG. 27 schematically shows motion of the meniscus 3 at the nozzle opening when the drive voltage waveform of FIG. 26(a) is applied. In the initial state when a reference voltage is applied, the meniscus 3 is flat as shown in FIG. 27(a). When the pressure generation chamber 100 is expanded by the first voltage change process 1 immediately before eject, the meniscus 3 is pulled backward as shown in FIG. 27(b). That is, the center of the meniscus 3 is recessed than the peripheral portion and a U-shaped meniscus 3 is formed. After the U-shaped meniscus 3 is formed, the pressure generation chamber 100 is compressed by the second voltage change process 2, so that a slender liquid column 4 is formed at the center of the meniscus 3 as shown in FIG. 27(c). Subsequently, the tip end of the liquid column 4 is separated to form an ink droplet 106 as shown in FIG. 27(d). Here, the ink droplet 106 has a diameter almost identical to the diameter of the liquid column 4, which is smaller than the diameter of the nozzle 101. Accordingly, this drive method enables to eject the ink droplet 106 having a smaller diameter than that of the nozzle 101. Hereinafter, the drive method for discharging a very small droplet by operating the meniscus 3 immediately before eject, that is the configuration of the ink droplet 3 reserved in the nozzle opening will be referred to as the meniscus control method.
As has been described above, by using the meniscus control method, it is possible to eject an ink droplet having a diameter smaller than the diameter of the nozzle. However, when using the drive voltage waveform as shown in FIG. 26(a), practically, the droplet diameter has a lower limit of 25 micrometers and it is impossible to satisfy the high quality image requirement.
The applicant of the present invention discloses in Japanese Patent Application 10-318443, a drive voltage waveform as shown in FIG. 26(b) as a drive method enabling to eject a further smaller droplet. This drive voltage waveform consists of a first voltage change process 1 for pulling a meniscus 3 toward the pressure generation chamber 100 immediately before eject, a second voltage change process 2 for compressing the volume of the pressure generation chamber 100 so as to form a liquid column for eject, a third voltage change process 5 for separating an ink droplet 106 quickly from the tip end of the liquid column 4, and a fourth voltage change process 6 for suppressing the residual pressure wave remaining after eject of the ink droplet. That is, the drive waveform of FIG. 26(b) includes the third voltage change process 5 for early separation of the ink droplet 106 and the fourth voltage change process 6 for suppressing reverberation in addition to the conventional meniscus control method as shown in FIG. 26(a). This enables to obtain a stable eject of the ink droplet 106 having a diameter in the order of 20 micrometers.
When discharging a very small droplet using the aforementioned meniscus control method, the greatest problem is to assure a stable eject. That is, the ink droplet diameter and eject speed of the ink droplet ejected by the meniscus control method greatly depend on the configuration of the meniscus 3 immediately before eject as shown in FIG. 27(b). Accordingly, in order to realize a stable eject, it is necessary to stabilize the configuration of the meniscus 3. Moreover, in the case of a multi-nozzle head having a plurality of nozzles, it is necessary to obtain identical meniscus configurations in the different nozzles. Practically, however, it is difficult to obtain identical meniscus configurations. As a result, irregularities are caused in the ink droplet diameter and droplet speed, deteriorating the image quality.
One of the causes which make the meniscus unstable and irregular is change of the initial meniscus configuration caused by an eject immediately before. Hereinafter, its mechanism will be explained with reference to FIG. 28.
When the ink droplet 106 is ejected from the nozzle 101, the amount of ink in the nozzle 101 is reduced and the meniscus 3 retreats toward the pressure generation chamber as shown in FIG. 28(a). The meniscus 3 which has retreated finally moves toward the nozzle opening plane as shown in FIG. 28(b) by the ink surface tension (capillary effect) so as to be ready for the next ejection. Such a recovery operation of the meniscus 3 is normally called refill operation.
In this refill operation, the meniscus 3 does not return directly to the still state of FIG. 28(b) from the state of FIG. 28(a). The meniscus is gradually converged to the still state while performing attenuation vibration around the nozzle opening plane. That is, the meniscus 3 which has retreated after ejection is restored to the nozzle opening plane as shown in FIG. 28(b) and overshoots to protrude from the nozzle opening plane as shown in FIG. 28(c) to form a convex meniscus 3. Then, the meniscus 3 again retreats to form a concave meniscus 3 as shown in FIG. 28(d). After repeating the convex and concave states, the meniscus gradually reaches the still state as shown in FIG. 28(b) or FIG. 28(f). The meniscus vibration cycle during this refill operation depends on the ink surface tension, the opening diameter of the nozzle 101, inertance of the fluid path system (nozzle, pressure generation chamber, ink supply path), and the like. Generally, the meniscus vibration cycle in an ordinary ink jet recording head is in the order of 80 to 150 seconds.
Here, what is important is the convex meniscus configuration caused by the overshoot of the meniscus 3. The overshoot of the meniscus 3 is especially remarkable in a head designed for high-speed recording. Moreover, the overshoot amount varies depending on the diameter of the droplet which has been ejected immediately before and the number of successive ejections. That is, in the case when an ejection has been performed immediately before, there the initial meniscus configuration for the following ejection may be of convex configuration, and the overshoot amount may not be constant. The applicant of the present invention has performed a number of ejection observation experiments and fluid analysis and found that the meniscus initial state of the convex configuration causes the stability of a very small droplet ejected by the meniscus control method to deteriorate. The mechanism will now be explained with reference to FIG. 29.
If the initial meniscus 3 has a convex configuration as shown in FIG. 29(a), the meniscus 3 is pulled in such a manner that the peripheral portion is pulled earlier than the center portion of the meniscus, which leads to the meniscus configuration as shown in FIG. 29(b). After that, as shown in FIG. 29(c), the center portion sinks partially. In this state, pressure for ejection is applied. Accordingly, normal liquid column formation cannot be performed. The ink droplet diameter and the droplet ejection speed are greatly changed. It should be noted that FIG. 29(d) shows abnormally slender liquid column 4, but this is not always the case when the initial meniscus is of convex configuration. For example, a slight difference in the meniscus configuration may greatly change the ejection phenomenon and the ejection speed may be greatly lowered in comparison to a normal ejection. That is, if the initial meniscus is of convex configuration, the droplet diameter and ejection speed fluctuate in a wide range. When a plurality of nozzles are used, irregularities between the nozzles are increased. Moreover, when an abnormal ejection phenomenon is caused as shown in FIG. 29, there also arises a problem that air bubbles are introduced into the nozzle, which causes a nozzle eject failure.
The aforementioned problem is especially severe when performing a droplet diameter modulation for changing the ink droplet diameter in multiple steps. That is, when performing a droplet diameter modulation, there is a case that a droplet of a large diameter is ejected immediately before discharging a very small droplet. The overshoot amount of the meniscus 3 increases as the droplet diameter increases. Accordingly, in this case, there is a high possibility that the initial meniscus has a convex configuration. This leads to great irregularities of the very small droplet diameter and ejection speed, remarkably deteriorating the image quality.
Moreover Japanese Patent Publication B53-12138 and Japanese Patent Publication A10-193587 disclose a so-called on-demand type ink jet recording apparatus.
With requirement for improvement of the recording image quality, in this type of ink jet recording head also, it is required to perform a high-quality recording. For this, it is necessary to express a smooth intermediate gradation.
For performing a gradation recording, there are two known methods. One of them uses a plurality of ink droplets of a fixed diameter to form a pixel (pseudo gradation), the other changes the ink droplet diameter in multiple steps for each bit.
In order to obtain a high quality image with the former method, it is necessary to highly increase the recording resolution. For this, the number of dots required for recording is greatly increased, causing a disadvantage that the recording speed is lowered.
On the other hand, the latter method can change concentration for each of the dots and enables to obtain a high image quality with a comparatively low recording resolution, which in turn enables to obtain a high recording speed.
Changing the ink droplet diameter in multiple steps can be realized by applying a plurality of drive voltage waveforms to the piezoelectric actuator 236 as shown in FIG. 33. FIG. 33 shows drive voltage waveforms for generating a small, intermediate, and large diameter of ink droplets. FIG. 33(a) is for the small diameter droplet, FIG. 33(b) is for the intermediate diameter droplet, and FIG. 33(c) is for the large diameter droplet. In FIG. 33(b) and FIG. 33(c), like portions as in the FIG. 33(a) are denoted by like reference symbols with a single or double quotation mark.
In FIG. 33, the pressure generation chamber 231 is expanded where the graph changes downward (portions indicated by 251 and 253) and the pressure generation chamber 231 is compressed where the graph changes upward (portions indicated by 252 and 254).
As shown in FIG. 33(c), if the pressure generation chamber 231 is slowly compressed taking a comparatively long time t3xe2x80x3, the compressed state of the pressure generation chamber 231 is maintained for a comparatively long time t4xe2x80x3, and the pressure generation chamber 231 is slowly expanded taking a comparatively long time t7xe2x80x3, then an ink droplet of a large diameter is ejected from the opening of the nozzle.
On the contrary, as shown in FIG. 33(a), if the pressure generation chamber 231 expanded is rapidly compressed taking a short time t3 and then rapidly expanded, an ink droplet of a small diameter is ejected from the opening of the nozzle.
FIG. 33(b) show a waveform that is in an ink ejecting state between that shown by FIGS. 33(a) and 33(c), that ejects an ink droplet of intermediate diameter is ejected from the opening of the nozzle.
The changing of the ink droplet diameter by changing the drive voltage waveform is disclosed as a so-called meniscus control method in the aforementioned Japanese Patent Publication A10-193587.
However, as has been described above, a number of pressure generation chambers are arranged in the ink jet recording head, and the piezoelectric actuator is also provided in the proximity. Accordingly, interference between the vibrations driven by the piezoelectric actuators makes it difficult to eject an ink droplet of a desired diameter.
Especially, as shown in FIG. 32, when adjacent piezoelectric actuators 236 are simultaneously driven (arrows in the figure indicate the vibration drive direction of the piezoelectric actuators), support members 237 for supporting the piezoelectric actuators 236 are deformed in the direction indicated by arrows. This deformation affects the pressure generation chambers 231 other than the corresponding one and causes a vibration loss. This results in irregularities of diameter and ejection speed of the ink droplets A, and is detrimental to obtaining a high quality recording image.
In order to solve this problem, i.e., the so-called cross talk, it is recommended to use a material of high rigidity for the members constituting the ink jet recording head such as piezoelectric actuators and pressure generation chambers, so as to eliminate affect of the piezoelectric actuators on the pressure generation chamber other than the corresponding one and to eliminate vibration loss.
However, forming an ink jet recording head from a material of high rigidity has various problems such as processing difficulty, increase of the ink jet recording head size, and increase of the production cost.
In Japanese Patent Publication A10-193587, the cross talk problem is solved by alternatively driving adjacent piezoelectric actuators. However, this leads to a problem that the recording time is prolonged.
Moreover as shown in FIG. 34, in this type of ink jet recording head, normally, one ink droplet reaching the recording medium forms one recording dot, and the dot size and the image quality are in inverse proportion. Accordingly, in order to satisfy the image quality, it is necessary to form a recording dot of a small diameter on the recording medium. In order to obtain a smooth image (high quality image) having no particle appearance for human eyes, the dot diameter should be 40 micrometers or below. If the dot diameter is 30 micrometers or below, the respective recording dots cannot be distinguished by visual observation even in a highlight portion of the image, and the image quality is by far improved.
The relationship between the ink droplet diameter and the dot diameter depends on the ink droplet flying speed, the ink properties (viscosity, surface tension), the type of the recording medium and the like. Normally, the dot diameter is about twice larger than the ink droplet. Accordingly, in order to obtain a dot diameter of 30 micrometers, the droplet diameter should be about 15 micrometers. It should be noted that in this Specification, an ink droplet diameter represents a total ink amount (including satellite) ejected by one ink droplet ejection, which amount is converted into a diameter of a sphere. Here, the satellite is a small secondary ink droplet formed together with an ink droplet.
On the other hand, experimentally it is known that the minimum value of the droplet diameter obtained from a nozzle having a predetermined opening diameter is almost equal to the opening diameter (nozzle diameter). Accordingly, in order to obtain a droplet of 15 micrometers, the nozzle diameter should be 15 micrometers or below. However, in order to make a nozzle having a diameter of 15 micrometers or below, various difficulties are involved in production and nozzle clogging is often caused. This significantly deteriorates the reliability and service life of the ink jet recording head. Accordingly, the nozzle diameter has a practical lower limit of 20 to 25 micrometers. Consequently, it has been difficult to obtain a stable ejection of ink droplets having a diameter of 15 micrometers or below. Moreover, if the nozzle diameter is reduced for reducing the ink droplet diameter, there arises a problem that a droplet of the maximum diameter for a desired resolution cannot be easily ejected.
In order to solve the aforementioned problem, for example, Japanese Patent Publication A55-17589 discloses an ink jet recording head drive method in which a drive waveform signal of reversed trapezoidal configuration as shown in FIG. 35 is applied to the piezoelectric actuator so as to perform the so-called meniscus control immediately before discharging an ink droplet, so as to eject an ink droplet having a diameter smaller than the nozzle diameter.
The drive waveform shown in FIG. 35 consists of a first voltage change process 308 for reducing to 0V for example, the voltage V which has been set to a reference voltage V1 ( greater than 0V) for application to the piezoelectric actuator; a voltage maintaining process 309 for maintaining the application voltage V which has been reduced to 0V for a certain period of time (time t2); and a voltage change process 310 for increasing the piezoelectric actuator application voltage V to the height of voltage V2, so as to reduce the volume of the pressure generation chamber to eject an ink droplet and to be ready for a subsequent eject operation.
It should be noted that the movement of the piezoelectric actuator by the increase or decrease of the voltage of the drive waveform signal depends on the configuration of the piezoelectric actuator and polarization direction. That is, there also exists a piezoelectric actuator moving in the reversed direction to the aforementioned piezoelectric actuator. For this piezoelectric actuator of the reversed movement, the voltage of the drive waveform signal can be reversed to obtain the same ejection operation as has been described above. For simplification, in this Specification, explanation will be given on a piezoelectric actuator which operates to reduce the volume of the pressure generation chamber when the voltage of the drive waveform signal is increased and to increase the volume of the pressure generation chamber when the voltage of the drive waveform signal is reduced.
FIG. 36 schematically shows movement of a meniscus 312 at the opening plane 311a of the nozzle 311 when the drive waveform signal shown in FIG. 35 is applied to the piezoelectric actuator. Firstly, when no ink droplet is to be ejected, as shown in FIG. 36(a), the meniscus 312 is at the opening plane 311a of the nozzle 311. When an ink droplet ejection is required, firstly, in order to increase the volume of the pressure generation chamber, the first voltage change process 308 of the drive waveform signal 1 is applied to the piezoelectric actuator. Then, as shown in FIG. 36(b), the meniscus 312 is pulled into the nozzle 311 from the opening plane 311a of the nozzle 311 and the meniscus configuration becomes concave (pulling process). After this, in order to reduce the volume of the pressure generation chamber, the second voltage change process 310 of the drive waveform signal is applied to the piezoelectric actuator. Then, as shown in FIG. 36(c), a liquid column 313 is formed at the center of the meniscus 312 and the tip end of the liquid column 313 is separated and as shown in FIG. 36(d), an ink droplet 314 is ejected (pushing process). The diameter of the ink droplet 314 ejected here is almost identical to the thickness of the liquid column 313 and smaller than the diameter of the nozzle 311.
However, in the conventional ink jet recording head drive method using the reversed trapezoidal drive waveform signal shown in FIG. 35, the ink droplet diameter actually obtained is about 25 micrometers at the smallest, which cannot satisfy the high quality request.
To cope with this, the inventor of the present invention has disclosed in Japanese Patent Application 10-318443 an ink jet recording head drive method in which a drive waveform signal having a waveform shown in FIG. 37 is applied to a piezoelectric actuator so as to eject a further small ink droplet.
The drive waveform signal shown in FIG. 37 consists of: a first voltage change process 315 for reducing the voltage V applied to the piezoelectric actuator from a reference voltage V1 ( greater than 0V) to 0V, so as to increase the volume of the pressure generation chamber and make the meniscus retreat; a first voltage maintaining process 316 for maintaining the voltage V reduced to 0 for a certain period of time (time t2); a second voltage change process 317 for increasing the piezoelectric actuator application voltage V to V2 so as to reduce the volume of the pressure generation chamber and to form a liquid column at the center of the meniscus; a second voltage maintaining process 318 for maintaining the voltage V2 for a certain period of time (time t4); a third voltage change process 319 for reducing the voltage V from V2 to 0V for example, so as to increase the volume of the pressure generation chamber and separate an ink droplet from the tip end of the liquid column; a third voltage maintaining process 320 for maintaining the application voltage V at 0V for a certain period of time (time t6); and a fourth voltage change process 321 for increasing the piezoelectric actuator application voltage V to voltage V1, so as to reduce the volume of the pressure generation chamber and suppress reverberation of the pressure wave remaining after the ink droplet eject.
That is, the drive waveform signal of FIG. 37 is a combination of the conventional meniscus control and an additional pressure wave control for early separation of an ink droplet and reverberation suppression. This enables stable ejection of an ink droplet having a diameter in the order of 20 micrometers.
However, in the conventional ink jet recording head drive method using the drive waveform signal having the waveform shown in FIG. 37, it is difficult to eject an ink droplet having a diameter smaller than 20 micrometers and it is impossible to eject an ink droplet of 15 micrometers or below.
To cope with this, the inventor of the present invention has disclosed in Japanese Patent Application 11-20613, an ink jet recording head drive method in which a drive waveform signal having a waveform shown in FIG. 38 is applied to the piezoelectric actuator, so as to eject an ink droplet having a diameter equal to or smaller than 15 micrometers.
The drive waveform signal shown in FIG. 38 consists of: a first voltage change process 322 for reducing the piezoelectric actuator application voltage V from a reference voltage Vb ( greater than 0V) to (Vbxe2x88x92V1) for a trailing time t1 which is greater than a natural period Ta of the natural vibration of a drive block consisting of a piezoelectric actuator and a diaphragm, so as to increase the volume of the pressure generation chamber and make the meniscus retreat; a first voltage maintaining process 323 for maintaining the voltage Vbxe2x88x92V1) for a certain period of time (time t2); a second voltage change process 324 for increasing the piezoelectric actuator application voltage V up to the voltage (Vbxe2x88x92V1+V2) for a trailing time t3 which is smaller than the natural period Ta, so as to reduce the volume of the pressure generation chamber and form a liquid column at the center of the meniscus; a second voltage maintaining process 325 for maintaining the application voltage V at the voltage (Vbxe2x88x92V1+V2) for a certain period of time (time t4); a third voltage change process 326 for reducing the application voltage V from the voltage (Vbxe2x88x92V1+V2) to 0V for example for a trailing time t5 which is smaller than the natural period Ta, so as to increase the volume of the pressure generation chamber and to separate an ink droplet from the liquid column at an early stage; a third voltage maintaining process 327 for maintaining the application voltage V at 0V for a certain period of time (time t6); and a fourth voltage change process 328 for increasing the piezoelectric actuator application voltage V up to the reference voltage Vb, so as to reduce the volume of the pressure generation chamber and suppress the reverberation of the pressure wave remaining after an ink droplet ejection.
That is, the drive waveform signal of FIG. 38 is a combination of the conventional meniscus control and a ejection mechanism utilizing the natural vibration of the piezoelectric actuator itself. Thus, the natural vibration of the piezoelectric actuator itself is excited and a high frequency vibration can be generated in the meniscus. This enables ejection of an ink droplet having a diameter of 15 micrometers or below.
However, in the conventional ink jet recording head drive method using the waveform shown in FIG. 38, the piezoelectric actuator deformation speed is increased. This significantly deteriorates the piezoelectric actuator reliability and service life.
Moreover, as has been described above, in order to excite the natural vibration of the piezoelectric actuator itself, it is necessary to change the voltage V applied to the piezoelectric actuator for a rise time t3 and trailing time t5 (1 microsecond for example) which are smaller than the natural period. In this case, a great current flows to the piezoelectric actuator instantaneously. Accordingly, the ink jet recording head drive circuit, especially, the piezoelectric actuator drive circuit should use a circuit part such as a semiconductor integrated circuit having a high current drive capability for instantaneously supplying a great current. Consequently, the circuit parts cost is increased, and a great current causes an increased heat dissipation, requiring a radiation unit. This increases the cost and size of the ink jet recording head drive circuit.
According to a first aspect of the present invention there is provided an ink jet recording head drive method and apparatus capable of stable ejection of a very small ink droplet by a meniscus control method to thereby achieve a high quality image.
An ink jet recording head drive method according to the first aspect of the present invention applies a drive voltage to an electro-mechanical converter which changes a pressure within a pressure generation chamber filled with ink, so that an ink droplet is ejected from a nozzle communicating with the pressure generation chamber, wherein the drive voltage has a voltage waveform including: a first voltage change process for increasing a volume of the pressure generation chamber so as to pull the ink meniscus from the nozzle opening toward the pressure generation chamber; and a second voltage change process for decreasing the volume of the pressure generation chamber, so as to eject the ink droplet, and wherein the first voltage change process is preceded by a preparatory voltage change process for slightly pulling the ink meniscus from the nozzle opening toward the pressure generation chamber.
That is, prior to the first voltage change process, the preparatory voltage change process is performed to slightly pull the ink meniscus at the nozzle opening toward the pressure generation chamber, so that the tip end of the meniscus is slightly pulled to the vicinity of the nozzle opening or to the pressure generation chamber. Thus, it is possible to obtain a stable and uniform initial meniscus state. This solves the various aforementioned problems.
Moreover, the preparatory voltage change process for slightly pulling the ink meniscus at the nozzle opening toward the pressure generation chamber prior to the first voltage change process can be realized by a preparatory voltage change process for increasing the volume of the pressure generation chamber. This voltage change process is to be performed prior to the first voltage change process, for stabilizing the meniscus configuration. Accordingly, its voltage change speed is preferably set at a smaller value than the voltage change speed of the first voltage change process, so that unnecessary vibration of meniscus is prevented.
Furthermore, in the preparatory voltage change process, by the same reason, the voltage change time of the voltage change process for increasing the volume of the pressure generation chamber is preferably set greater (longer) than the natural period of the pressure wave generated in the pressure generation chamber.
It should be noted that when the volume of the pressure generation chamber is increased, prior to the first voltage change process, so that the meniscus is slightly pulled toward the pressure generation chamber, the meniscus at the nozzle opening plane or retrieved from the nozzle opening plane upon completion of the preceding ejection is further pulled toward the pressure generation chamber. The applicant of the present invention has confirmed that a slight retrieval of the meniscus from the nozzle opening plane does not cause a large fluctuation of the droplet diameter or the droplet speed.
Moreover, the preparatory voltage change process for slightly pulling the ink meniscus at the nozzle opening toward the pressure generation chamber prior to the first voltage change process can be realized by a preparatory voltage change process consisting of a voltage change process for decreasing the volume of the pressure generation chamber and a voltage maintaining process for maintaining the voltage for a predetermined period of time.
In this method, firstly, the volume of the pressure generation chamber is decreased to cause a temporal overshoot state of the meniscus. However, while the voltage is maintained for the predetermined period of time, the meniscus overshoot state naturally disappears by the ink surface tension. In the same way as when the volume of the pressure generation chamber is increased prior to the first voltage change process, it is possible to obtain a stable and uniform initial meniscus configuration at the start of the first voltage change process.
In this case also, in order to stabilize the meniscus configuration earlier, by preventing a sudden overshoot generation and vibration, the voltage change time of the voltage change process, in the preparatory voltage change process, for decreasing the pressure generation chamber volume is preferably set greater (longer) than the natural period of the pressure wave generated in the pressure generation chamber.
Furthermore, duration of the voltage maintaining process following the voltage change process for decreasing the pressure generation chamber volume is optimally set at ⅓ to ⅔ of the natural period of vibration of the ink droplet at the nozzle opening, i.e., the natural period of the attenuation vibration of the meniscus.
Thus, even if the meniscus protrudes by overshoot at the final stage of the voltage change process for decreasing the pressure generation chamber volume, the aforementioned first voltage change process can be started at the trough of the amplitude generated by attenuation vibration, i.e., at the meniscus retrieved from the nozzle surface as the initial state.
Moreover, when the present invention is applied to an apparatus, one or more than one waveform generation unit for generating a drive voltage to be applied to an electro-mechanical converter include a function to generate a waveform having the preparatory voltage change process for slightly pulling an ink meniscus toward the pressure generation chamber prior to the first voltage change process.
The electro-mechanical converter may be a piezoelectric actuator.
According to a second aspect of the present invention, there is provided an ink jet recording head drive method and drive apparatus which solves the structural problem of cross talk in the ink jet recording head without lowering the printing speed and enables both high quality and a high speed recording.
An ink jet recording head drive method according to the second aspect the present invention provides an ink jet recording head comprising: a plurality of pressure generation chambers filled with ink; nozzles provided in the pressure generation chambers for discharging the ink; and a vibration generation unit provided for each of the pressure generation chambers that causes a pressure change in the respective pressure generation chamber, wherein drive voltage waveforms to be applied to the vibration generation units are prepared according to a diameter of ink droplets to be ejected, so that the drive voltage waveforms corresponding to different ink droplet diameters are applied at predetermined different timings.
Further according to the method of the second aspect, drive voltage waveforms are generated according to droplet diameters and the drive voltage waveforms are applied to vibration generation unit provided for each of the pressure generation chambers, at predetermined different timings. Accordingly, when an ink droplet is ejected from one of the pressure generation chambers, the vibration will not affect the other pressure generation chambers. Thus, an ink droplet of a desired diameter can be generated in each of the pressure generation chambers and ejected from a nozzle at a desired speed.
Moreover, since the drive voltage waveforms are generated according to the ink diameters, it is possible to successively eject ink droplets of different diameters within a short period of time, without prolonging time required for recording.
According to the second aspect of the present invention, the drive voltage waveforms are set so that a smaller diameter ink droplet is ejected earlier.
As the ink droplet becomes smaller, i.e., the mass becomes smaller, the air resistance becomes greater and it takes more time to reach a recording medium. According to this method, a droplet of smaller diameter is ejected earlier. This reduces the difference in time to reach the recording medium, which improves the recording image quality.
Further according to the second aspect of the present invention, the drive voltage waveform for discharging a small diameter ink droplet includes a portion for pulling the meniscus at the nozzle toward the pressure generation chamber.
According to this method, it is possible to obtain an ink droplet of a desired diameter with a high accuracy, which enables obtaining a recorded image of a high quality.
An apparatus according to a second aspect the present invention, is comprised of an ink jet recording head drive apparatus for an ink jet recording head including: a plurality of pressure generation chambers; nozzles provided to communicate with the pressure generation chambers for discharging ink; and vibration generation units provided for generating vibration to cause an inner pressure change in the pressure generation chambers wherein a drive voltage waveforms are applied to the vibration generation unit for discharging ink droplets from the nozzle, the apparatus comprising a plurality of waveform generation units provided according to the diameter of ink droplets to be ejected, so as to generate drive voltage waveforms according to the ink droplet diameter, wherein the drive voltage waveforms generated according to the ink droplet diameter by the waveform generation unit are set so as to be generated at different ejection times according to the different ink droplet diameters.
According to this configuration, drive voltage waveforms are generated according to the ink droplet diameters, and the drive voltage waveform are applied, with different timing, to the vibration generation unit provided for the respective pressure generation chambers. Accordingly, when an ink droplet is ejected from a pressure generation chamber, the vibration will not affect the other pressure generation chambers. Thus, an ink droplet of a desired diameter can be obtained in each of the pressure generation chambers and ejected from the nozzle at a desired speed.
Moreover, since the drive voltage waveforms are generated according to the different diameters of ink droplets, it is possible to successively eject ink droplets of different diameters within a short period of time without prolonging the time required for recording.
Further according to the second aspect of the present invention, the vibration generation unit is a piezoelectric actuator.
This enables to reduce the apparatus size and control the pressure wave generation in the pressure generation chamber with a high accuracy.
Further according to the second aspect of the present invention, the piezoelectric actuator generates a longitudinal vibration.
By using the piezoelectric actuator of longitudinal vibration type, it is possible to reduce the size of the actuator in comparison to the actuator of deflection vibration type, which in turn enables a high density arrangement of nozzles.
According to a third aspect of the present invention, there is provided an ink jet recording head drive method and a circuit thereof capable of discharging a small ink droplet having a diameter equal to or smaller than 20 micrometers without deteriorating the reliability and service life of the piezoelectric actuator, and at a reasonable cost and with a small size configuration.
With a view to solving the above-mentioned problem, an ink jet recording head drive method according to the third aspect of the invention applies to an ink jet recording head comprising a pressure generation chamber filled with ink, a pressure generation unit for generating a pressure in the pressure generation chamber, and a nozzle communicating with the pressure generation chamber, wherein a drive waveform signal is applied to the pressure generation unit so as to change the volume of the pressure generation chamber so that an ink droplet is ejected from the nozzle, the drive waveform signal having a waveform including at least: a first voltage change process for applying a voltage in the direction to increase the volume of the pressure generation chamber; and a second voltage change process for applying a voltage in the direction to decrease the volume of the pressure generation chamber, wherein the first voltage change process has a voltage change time set within a range of about ⅓ to ⅔ of a natural period TC of a pressure wave generated in the pressure generation chamber, and the second voltage change process has a start time set immediately after completion of the first voltage change process.
Moreover, an ink jet recording head drive method according to the third aspect of the invention is characterized in that the first voltage change process in the waveform of the drive waveform signal has a voltage change time set to xc2xd of the natural period TC.
Moreover, an ink jet recording head drive method according to the third aspect of the invention is characterized in that the waveform of the drive waveform signal is such that a time interval between the end time of the first voltage change process and the start time of the second voltage change process is set to a length equal to or shorter than about ⅕ of the natural period TC.
Moreover, an ink jet recording head drive method according to the third aspect of the invention is characterized in that the waveform of the drive waveform signal is such that the second voltage change process has a voltage change time set to about ⅓ of the natural period TC or below.
Moreover, an ink jet recording head drive method further according to the third aspect of the invention is characterized in that the waveform of the drive waveform signal is such that the second voltage change process is followed by a third voltage change process for applying a voltage in the direction to increase the volume of the pressure generation chamber.
Moreover, an ink jet recording head drive method according to a first variation of the third aspect of the invention is characterized in that the waveform of the drive waveform signal is such that the third voltage change process has a voltage change time set to about ⅓ of the natural period TC.
Moreover, an ink jet recording head drive method according to a second variation of the third aspect of the invention is characterized in that the waveform of the drive waveform signal is such that a time interval between the second voltage change process end time and the third voltage change process start time is set to about ⅕ of the natural period TC or below.
Moreover, an ink jet recording head drive method according to a third variation of the third aspect of the invention is characterized in that the waveform of the drive waveform signal is such that the third voltage change process has a voltage change amount set to be greater than the voltage change amount of the second voltage change process.
Moreover, an ink jet recording head drive method according to a fourth variation of the third aspect of the invention is characterized in that the waveform of the drive waveform signal is such that the third voltage change process is followed by a fourth voltage change process for applying voltage in the direction to reduce the volume of the pressure generation chamber.
Moreover, an ink jet recording head drive method according to a first variant of the fourth variation of the third aspect of the invention is characterized in that the drive waveform signal has a such a waveform that the fourth voltage change process has a voltage change time set to about xc2xd of the natural period TC or below.
Moreover, an ink jet recording head drive method according to a second variant of the fourth variation of the third aspect of the invention is characterized in that the drive waveform signal has a such a waveform that the time interval between the end of the third voltage change process and the start time of the fourth voltage change process is set to about ⅓ of the natural period TC or below.
Moreover, an ink jet recording head drive method according to a fifth variation of the third aspect of the invention is characterized in that the natural period TC is 15 microseconds or below.
Moreover, an ink jet recording head drive method according to a sixth variation of the third aspect of the invention is characterized in that the pressure generation unit is an electro-mechanical converter.
Moreover, an ink jet recording head drive method according to a first variant of the sixth variation of the third aspect of the invention is characterized in that the electro-mechanical converter is a piezoelectric actuator.
An ink jet recording head drive circuit for an ink jet recording head according to the third aspect of the invention comprises a pressure generation chamber filled with ink, pressure generation unit for generating a pressure in the pressure generation chamber, and a nozzle communicating with the pressure generation chamber, wherein a drive waveform signal is applied to the pressure generation unit so as to change the volume of the pressure generation chamber so that an ink droplet is ejected from the nozzle, the circuit comprising a waveform generation unit operating according to a drive waveform signal having a waveform consisting of at least: a first voltage change process for applying a voltage in the direction to increase the volume of the pressure generation chamber; and a second voltage change process for applying a voltage in the direction to decrease the volume of the pressure generation chamber, wherein the first voltage change process has a voltage change time set within a range of about ⅓ to ⅔ of a natural period TC of a pressure wave generated in the pressure generation chamber, and the second voltage change process has a start time set immediately after completion of the first voltage change process.
Moreover, an ink jet recording head drive circuit according to the third aspect of the invention, is further characterized in that said waveform generation unit generates a drive waveform signal having a waveform in which the voltage change time of the first voltage change process is set to about xc2xd of the natural period TC.
Moreover, a first variation of an ink jet recording head drive circuit according to the third aspect of the invention is further characterized in that said waveform generation unit generates a drive waveform signal having a waveform in which the time interval between the end time of the first voltage change process and the start time of the second voltage change process is set to about ⅕ of the natural period or below.
Moreover, a second variation of an ink jet recording head drive circuit according to the third aspect of the invention is further characterized in that the waveform generation unit generates such a drive waveform signal that the second voltage change process has a voltage change time set to about ⅓ of the natural period TC or below.
Moreover, a third variation of an ink jet recording head drive circuit according to the third aspect of the invention is further characterized in that the waveform generation unit generates such a drive waveform signal that the second voltage change process is followed by a third voltage change process for applying a voltage in the direction to increase the volume of the pressure generation chamber.
Moreover, a first variant of the third variation of an ink jet recording head drive circuit according to a third aspect of the invention is characterized in that the waveform generation unit generates a drive waveform signal having such a waveform that the third voltage change process has a voltage change time set to about ⅓ of the natural period TC.
Moreover, a second variant of the third variation of an ink jet recording head drive circuit according to the third aspect of the invention is characterized in that the waveform generation unit generates a drive waveform signal having is such waveform that a time interval between the second voltage change process end time and the third voltage change process start time is set to about ⅕ of the natural period TC or below.
Moreover, a third variant of the third variation of an ink jet recording head drive circuit according to the third aspect of the invention is characterized in that the waveform generation unit generates a drive waveform signal having such a waveform that the third voltage change process has a voltage change amount set to be greater than the voltage change amount of the second voltage change process.
Moreover, a fourth variant of the third variation of an ink jet recording head drive circuit according to the third aspect of the invention is characterized in that the waveform generation unit generates a drive waveform signal having such a waveform that the third voltage change process is followed by a fourth voltage change process for applying voltage in the direction to reduce the volume of the pressure generation chamber.
Moreover, a first variant of the fourth variant of the third variation of an ink jet recording head drive circuit according to the third aspect of the invention is characterized in that the waveform generation unit generates a drive waveform signal having a such a waveform that the fourth voltage change process has a voltage change time set to about xc2xd of the natural period TC or below.
Moreover, a second variant of the fourth variant of the third variation of an ink jet recording head drive circuit according to the third aspect of the invention is characterized in that the waveform generation unit generates a drive waveform signal having a such a waveform that the time interval between the end of the third voltage change process and the start time of the fourth voltage change process is set to about ⅓ of the natural period TC or below.
Moreover, a fourth variation of an ink jet recording head drive circuit according to the third aspect of the invention is characterized in that the natural period TC is 15 microseconds or below.
Moreover, a fifth variation of an ink jet recording head drive circuit according to the third aspect of the invention is characterized in that the pressure generation unit is an electro-mechanical converter.
Moreover, a first variant of the fifth variation of an ink jet recording head drive circuit according to the third aspect of the invention is characterized in that the electro-mechanical converter is a piezoelectric actuator.
According to the present invention, it is possible to eject a small ink droplet having a diameter of 20 micrometers or below without deteriorating the piezoelectric actuator reliability and service life, and with a small size configuration at a low cost.
Before describing the invention in detail, an explanation will be given on a theoretical basis of the validity of the present invention using a lumped parameter circuit model.
FIG. 20(a) is circuit diagram equivalent to the ink jet recording head filled with ink shown in FIG. 12(a). In FIG. 20, m0 represents inertance (acoustic mass) [kg/m4] of a drive block consisting of a piezoelectric actuator 336 and a diaphragm 335; m2 represents inertance of an ink supply hole 333; m3 represents inertance of a nozzle 334; r0 represents acoustic resistance of the drive block [Ns/m5]; r2 represents acoustic resistance of the ink supply hole 333; r3 represents acoustic resistance of the nozzle 334; c0 represents acoustic capacity [m5/N] of the drive block; c1 represents acoustic capacity of the pressure generation chamber 331; c3 represents acoustic capacity of the nozzle 334; u1 represents volume velocity in the ink supply hole 333; u2, volume velocity in the ink supply hole 333; u3 represents volume velocity in the nozzle 334; and xcex8 represents pressure [Pa] applied to the ink.
Here, if the piezoelectric actuator 336 is a highly-rigid layered type piezoelectric actuator, it is possible to ignore the drive block inertance m0, the acoustic resistance r0, and the acoustic capacity c0. Moreover, when analyzing a pressure wave, it is also possible to ignore the acoustic capacity c3. Accordingly, the equivalent circuit of FIG. 20(a) can approximately be represented by an equivalent circuit of FIG. 20(b).
Moreover, assuming that the intertances m2 and m3 of the ink supply hole 333 and the nozzle 334 are in the relationship of m2=km3 and that the acoustic resistances r2 and r3 of the ink supply hole 333 and the nozzle 334 are in the relationship of r2=kr3, and if a drive waveform signal having a rise angle of xcex8 is input for circuit analysis as shown in FIG. 21(a), a particle velocity (velocity of ink molecule) V3xe2x80x2 [m/s] in the nozzle 334 within the rise time 0xe2x89xa6txe2x89xa6t1 is given by Equation (1). In Equation (1), A3 represents an area of the opening of the nozzle 334, and the particle velocity (velocity of ink molecule) V3xe2x80x2 in the nozzle 334 is an volume velocity u3 in the nozzle 334 divided by the area A3 of the opening of the nozzle 334.                                                         v              3              xe2x80x2                        ⁡                          (                              t                ,                θ                            )                                =                                                                      c                  1                                ⁢                tan                ⁢                                  xe2x80x83                                ⁢                θ                                                              A                  3                                ⁡                                  (                                      1                    +                                          1                      k                                                        )                                                      ⁡                          [                              1                -                                                      W                                          E                      c                                                        ⁢                                      exp                    ⁡                                          (                                                                        -                                                      D                            c                                                                          ·                        t                                            )                                                        ⁢                                      sin                    ⁡                                          (                                                                                                    E                            c                                                    ·                          t                                                -                                                  φ                          0                                                                    )                                                                                  ]                                      ⁢                  
                ⁢                              E            c                    =                                                                      1                  +                                      1                    k                                                                                        c                    1                                    ⁢                                      m                    3                                                              -                              D                c                2                                                    ⁢                  
                ⁢                              D            c                    =                                    r              3                                      2              ⁢                              m                3                                                    ⁢                  
                ⁢                              w            2                    =                                    1              +                              1                k                                                                    c                1                            ⁢                              m                3                                                    ⁢                  
                ⁢                              φ            0                    =                                    tan                              -                1                                      ⁡                          (                                                E                  c                                                  D                  c                                            )                                                          (        1        )            
Next, when using a drive waveform signal of a complicated (trapezoidal) configuration as shown in FIG. 21(b), the particle velocity can be obtained by superimposing a pressure wave generated at the turning points (A, B, C, D) of the drive waveform signal. That is, when the drive waveform signal of FIG. 21(b) is used, the particle velocity V3 [m/s] in the nozzle 334 can be given by Equation (2).                                                                                                                                                                                       v                          3                                                ⁡                                                  (                          t                          )                                                                    =                                              xe2x80x83                                            ⁢                                                                                                    v                            3                            xe2x80x2                                                    ⁡                                                      (                                                          t                              ,                                                              θ                                1                                                                                      )                                                                          ⁢                                                  xe2x80x83                                                ⁢                                                  (                                                      0                            ≤                            t                             less than                                                           t                              1                                                                                )                                                                                                                                                                                                                                  v                          3                                                ⁡                                                  (                          t                          )                                                                    =                                              xe2x80x83                                            ⁢                                                                                                    v                            3                            xe2x80x2                                                    ⁡                                                      (                                                          t                              ,                                                              θ                                1                                                                                      )                                                                          +                                                                                                            v                              3                              xe2x80x2                                                        ⁡                                                          (                                                                                                t                                  -                                                                      t                                    1                                                                                                  ,                                                                  θ                                  2                                                                                            )                                                                                ⁢                                                      xe2x80x83                                                    ⁢                                                      (                                                                                          t                                1                                                            ≤                              t                               less than                                                                                                 t                                  1                                                                +                                                                  t                                  2                                                                                                                      )                                                                                                                                                                                                                                                            v                          3                                                ⁡                                                  (                          t                          )                                                                    =                                              xe2x80x83                                            ⁢                                                                                                    v                            3                            xe2x80x2                                                    ⁡                                                      (                                                          t                              ,                                                              θ                                1                                                                                      )                                                                          +                                                                              v                            3                            xe2x80x2                                                    ⁡                                                      (                                                                                          t                                -                                                                  t                                  1                                                                                            ,                                                              θ                                2                                                                                      )                                                                          +                                                                                                                                                                                xe2x80x83                                            ⁢                                                                                                    v                            3                            xe2x80x2                                                    ⁡                                                      (                                                                                          t                                -                                                                  t                                  1                                                                -                                                                  t                                  2                                                                                            ,                                                              θ                                3                                                                                      )                                                                          ⁢                                                  xe2x80x83                                                ⁢                                                  (                                                                                                                    t                                1                                                            +                                                              t                                2                                                                                      ≤                            t                             less than                                                                                           t                                1                                                            +                                                              t                                2                                                            +                                                              t                                3                                                                                                              )                                                                    "AutoRightMatch"                                                                                                                                                                                                                                                            v                          3                                                ⁡                                                  (                          t                          )                                                                    =                                              xe2x80x83                                            ⁢                                                                                                    v                            3                            xe2x80x2                                                    ⁡                                                      (                                                          t                              ,                                                              θ                                1                                                                                      )                                                                          +                                                                              v                            3                            xe2x80x2                                                    ⁡                                                      (                                                                                          t                                -                                                                  t                                  1                                                                                            ,                                                              θ                                2                                                                                      )                                                                          +                                                                                                                                                                                xe2x80x83                                            ⁢                                                                                                    v                            3                            xe2x80x2                                                    ⁡                                                      (                                                                                          t                                -                                                                  t                                  1                                                                -                                                                  t                                  2                                                                                            ,                                                              θ                                3                                                                                      )                                                                          +                                            ⁢                                              xe2x80x83                                                                                                                                                                                xe2x80x83                                            ⁢                                                                                                    v                            3                            xe2x80x2                                                    ⁡                                                      (                                                                                          t                                -                                                                  t                                  1                                                                -                                                                  t                                  2                                                                -                                                                  t                                  3                                                                                            ,                                                              θ                                4                                                                                      )                                                                          ⁢                                                  xe2x80x83                                                ⁢                                                  (                                                      t                            ≥                                                                                          t                                1                                                            +                                                              t                                2                                                            +                                                              t                                3                                                                                                              )                                                                    ⁢                                              xe2x80x83                                                                                                                                }                            (        2        )            
Here, FIG. 23 shows a particle velocity change according to time when a drive waveform signal of FIG. 22 is used, the change being calculated by using Equation (2) considering only a vibration component of Equation (1). The drive waveform signal shown in FIG. 22 consists of a first voltage change process 341 for reducing the piezoelectric actuator application voltage from a reference voltage V1 ( greater than 0V) to 0V for example, so as to increase the volume of the pressure generation chamber and make the meniscus retreat; a voltage maintaining process 342 for maintaining the application voltage V at 0V for a certain period of time (time t2); and a second voltage change process 343 for increasing the piezoelectric actuator application voltage V to V2, so as to reduce the volume of the pressure generation chamber, eject an ink droplet, and be ready for the subsequent eject operation.
In FIGS. 23(a) and 23(b), thin lines xe2x80x9caxe2x80x9d to xe2x80x9cdxe2x80x9d represent particle velocity change at the turning points A, B, C, and D of the drive waveform signal shown in FIG. 22, and the thick line xe2x80x9csxe2x80x9d represents a sum of the particle velocities, i.e., particle velocity change according to the time actually generated in the meniscus.
(1) In the drive waveform signal shown in FIG. 22, when t1 is set as xc2xd of the natural period Tc (=2/Ec) of the pressure wave generated in the pressure generation chamber and t2 is set to a very small value, as shown in FIG. 23(a), the time change phases of the particle velocity at the turning points A, B, and C are almost matched with one another. Accordingly, in the time interval (t greater than t1+t2), the particle velocity is suddenly increased.
Next, explanation will be given on the meniscus configuration change when such a sudden change has occurred in the particle velocity with reference to FIG. 23 and FIG. 24.
When the particle velocity change shown in FIG. 23(a) is applied to the meniscus 354, within the time t1, the meniscus 354 is pulled from the opening plane of the nozzle 351 into the nozzle 351 and becomes concave. Next, within the time t2, the meniscus 354 is pushed out of the nozzle 351. When push is applied to the concave configuration of the meniscus 354 is pushed out of the nozzle 351. When push is applied to the concave configuration of the meniscus 354, a slender liquid column 352 is formed at the center of the meniscus 354.
There has been no detailed study about the formation mechanism of this liquid column 352. The inventor of the present invention performed observation of the ink droplet ejection and fluid analysis and confirmed that the thickness of the liquid column 352 depends on the velocity of the liquid surface when the meniscus 354 is pushed out. That is, when a push out force is applied to the concave meniscus 354, as shown in FIGS. 24(a) and 24(b), each of the meniscus 354 portions moves in the direction of the normal lines (arrows in the figures). As a result, a large amount of ink is concentrated in the center of the nozzle 351. This local ink volume increase forms the liquid column 352 at the center of the nozzle 351. Here, if the liquid surface movement velocity is high, the ink volume is also rapidly increased at the center of the nozzle 351 and accordingly, a very slender liquid column 352 is rapidly formed (see FIG. 24(a)). Conversely, when the liquid surface movement velocity is low, the ink volume increase at the center of the nozzle 351 becomes also slow and accordingly, the liquid column 352 becomes thicker and the column growth also becomes slow (see FIG. 24(b)).
It should be noted that, as has been described above, the diameter of the ink droplet 353 ejected from the nozzle 351 using the xe2x80x9cmeniscus controlxe2x80x9d method is almost identical to the thickness of the liquid column 352 formed. Moreover, the ink droplet flying velocity (droplet velocity) is almost identical to the growth velocity of the liquid column 352.
Accordingly, in order to eject a small ink droplet at a high speed, it is necessary to increase the liquid surface movement velocity at the xe2x80x9cpushxe2x80x9d process to cause a rapid ink volume increase at the center of the nozzle 351.
Based on the aforementioned observation, in the drive waveform signal of FIG. 22, the conditions of time t1 set to xc2xd of the natural period Tc and the time t2 set to a very small value are significantly advantageous for discharging a small ink droplet. That is, under such a condition, as shown in FIG. 23(a), the time change phases of the particle velocities at the turning points A, B, and C are almost overlapped. Accordingly, within a time interval (t greater than t1+t2), the particle velocity is suddenly increased and the liquid surface movement velocity becomes high. This causes a rapid ink volume increase at the center of the nozzle 351, which forms a slender liquid column 352. As a result, a very small ink droplet 353 can be ejected at a high speed. That is, the sudden increase of the liquid surface movement velocity of the meniscus 354 is an important condition for discharging the very small ink droplet 353.
(2) On the other hand, in the drive waveform signal shown in FIG. 22, if the time t1 has not been set to xc2xd of the natural period TC, the time change phases of the particle velocities at the turning points A, B, and C are not matched as shown in FIG. 23(b) and the sum (thick line s) of the particle velocities becomes a dull change.
That is, if the time t1, is shorter than xc2xd of the natural period TC, while the particle velocity generated at the turning point A is negative, a positive particle velocity is generated at the turning point B. These velocities cancel each other, and the increase of the movement velocity of the liquid surface of the meniscus 354 becomes dull. On the other hand if the time t1 is longer than xc2xd of the natural period TC, the particle velocity generated at the turning point A becomes positive before generation of the positive particle velocity at the turning point B. In this case also, it is impossible to obtain a rapid increase of the liquid surface movement velocity of the meniscus 354.
Under these conditions, it becomes difficult to obtain a rapid ink volume increase at the center of the nozzle 351, and the liquid column 352 becomes thicker. As a result, the diameter of the ink droplet 353 ejected becomes larger and the droplet velocity becomes slower (see FIG. 24(b)). Thus, it becomes impossible to obtain a very small ink droplet having a diameter of 20 micrometers or below required for high quality recording.
As has been described above, the droplet diameter and the droplet velocity of the ink droplet 353 ejected from the nozzle 351 greatly depend on the voltage change time t1 of the first voltage change process 341 and the voltage maintaining time t2, i.e., a time interval between the end time of the first voltage change process 341 and the start time of the second voltage change process 343 in the drive waveform signal shown in FIG. 22. By setting the voltage change time t1 at about xc2xd of the natural period TC and setting the voltage maintaining time t2 at a sufficiently short value, it is possible to eject a very small ink droplet at a high velocity.
It should be noted that in this case, because the natural vibration of the piezoelectric actuator itself is not utilized, there is no danger of deteriorating the reliability and the service life of the piezoelectric actuator. Moreover, the drive circuit of the ink jet recording head, especially the drive circuit of the piezoelectric actuator is identical to the conventional configuration and accordingly, there is no need of increase the production cost and size of the ink jet recording head drive circuit.